1. Field of the Invention
The present invention relates to a focus condition detecting device for a camera which detects the focus condition of an objective lens of a camera via object light which had passed through the object lens.
2. Description of the Prior Art
There has been known a focus condition detecting device in which two images are formed by refocusing light bundles of object light which have passed through a first and a second area of an objective lens, each area being symmetric with respect to the optical axis of the objective lens. The relative distance between these two refocused images is calculated and the defocus amount of the focus position from a predetermined focus position and the direction of the defocus are determined based on the relative distance calculated.
A typical optical system for such a focus condition detecting device as mentioned above is shown in FIG. 1.
In this system, a condenser lens 6 is usually arranged on a predetermined focal plane 4 of an objective lens 2 or on a plane positioned behind the focal plane and two refocusing lenses 8 and 10 are arranged behind the condenser lens 6. There are arranged two image sensors 12 and 14 on each of the focal planes of two refocusing lenses. Each of the image sensors 12 and 14 is comprised of a CCD (charge coupled device) image sensor having a plurality of light sensing elements.
The condenser lens 6 is so designed that the effective ranges of the first and second refocusing lenses 8 and 10 can be extended from the exit pupil of the objective lens in order to form first and second images with the light bundles which have passed through the exit pupil and first and second refocusing lenses.
As shown in FIG. 2 schematically, when an image (A) of an object is focused in front of the predetermined focal plane 4, two images a and a' are formed on the image sensors 12 and 14 relatively close to each other with respect to the optical axis 18 of the objective lens. On the other hand, when an image (B) of an object is focused to the rear of the predetermined focal plane 4, two images b, b' are formed further apart from each other. If an image is focused just on the predetermined focal plane 4, a distance between two corresponding points of the two images formed on two image sensors 12 and 14 becomes a specific value which is determined by the composition of the optical system of the focus condition detecting device. Accordingly, a focus condition of the objective lens can be determined from the distance between two corresponding points of the two images formed on the image sensors 12 and 14.
The detection of the distance mentioned above can be made as follows:
FIG. 3 shows a principle therefor schematically. As shown therein, the first and second image sensors 12 and 14 are comprised of ten and sixteen cells of photodiodes (a.sub.1, . . . , a.sub.10) and (b.sub.1, . . . ,b.sub.16), respectively. Now, assume that each symbol of individual cell represents each output thereof. Considering now sets, each of which is comprised of ten successive cells included in the second image sensor, seven sets B.sub.1, b.sub.2, . . . B.sub.7 can be obtained as shown in FIG. 3. The focus condition can be sought by calculating individual correlation relation between the image received by ten cells of the first image sensor 12 and the image received by each of seven sets of the second image sensor 14.
Namely, correlation calculations are made with use of correlation functions: ##EQU1##
For example, if the image detected by the first sensor 12(A.sub.1) coincides with the image detected by the first set B.sub.1 of the second image sensor 14, the correlation function S.sub.1 becomes minimum among the seven correlation functions S.sub.1, S.sub.2, . . , S.sub.7. When a set (S.sub.i) of the second sensor is found which gives the minimum value with respect to these correlation functions, the distance between the two images is determined from the identity of the set having been found to yield the minimum, and a focus condition is detected based on the distance determined. These calculations are carried out by a correlation calculation means 16.
In FIG. 4 showing a scene viewed through a viewfinder of a camera, the focus detection area A is limited to a relatively narrow center area of the field of view of the objective lens. In this case, the image of one person (main object) existing near the camera and the background image comprised of trees and a hill are entered into the focus detection area A and, due to this situation, the focus condition detecting device is liable to detect an intermediate distance between the person and the background as a correct distance of the object.
Meanwhile, in the case wherein a few objects having different distances from the camera respectively are present in the focus detection area, the main object usually exists at a position nearer than those of the subjects (background) and, usually, may be one or more persons. The subobject is usually a background of the person. Considering such an ordinary or typical object as mentioned above, the image of the main object such as a person or persons usually has a contrast higher than that of the background.
In Japanese patent laid-open publication No. 126517/1984, there is proposed a focus detecting device in which the focus detection area is divided into a plurality of blocks and a focus detection operation is carried out in every block, the credibility of the individual focus detecting operation in every block is checked, and the result obtained through the focus detecting operation having the highest credibility is employed as the focus detection area.
However, according to this focus detecting device, a long calculation time is needed since the credibility of focus condition detection has to be checked for every block.